<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>

</head>

<body>
  <div id="app">
      firstName: 
      <input type="text" v-model="firstName">
      lastName:
      <input type="text" v-model="lastName">
      fullName:
      =>
      {{fullName}}
  </div>
  <script src="./vue.global.js"></script>
  <script>
    const { createApp } = Vue;
    const app = createApp({
      data() {
        return {
          firstName:'',
          lastName: '',
          fullName: ''
        }
      },
      watch: {
        firstName(val){
          this.fullName = val + '.' + this.lastName
        },
        lastName(val){
          this.fullName = this.firstName + '.' + val
        }
      }
    })
    app.mount('#app')
  </script>
</body>

</html>